Kotlin 2 cheatsheet

Kotlin 2: Classes and types

7 common Kotlin 2 tasks.

Data classes

A data class gets equals, hashCode, toString, copy and destructuring for free.

data class User(val id: Int, val name: String)

val ada = User(1, "Ada")
val renamed = ada.copy(name = "Ada Lovelace")
val (id, name) = ada

Classes and constructors

The primary constructor is part of the class header. Properties can have getters.

class Account(val owner: String, balance: Int = 0) {
    var balance = balance
        private set

    fun deposit(amount: Int) { balance += amount }
}

Sealed types for fixed alternatives

A sealed interface limits its implementations, so when must handle every case.

sealed interface Result {
    data class Success(val value: Int) : Result
    data class Failure(val message: String) : Result
}

fun show(r: Result) = when (r) {
    is Result.Success -> "ok ${r.value}"
    is Result.Failure -> "error ${r.message}"
}

Singletons and companion objects

object declares a singleton, and a companion object holds class-level members.

object Config { const val VERSION = "1.0" }

class User private constructor(val name: String) {
    companion object {
        fun create(name: String) = User(name.trim())
    }
}

Add a function to an existing type

An extension function is called like a member but defined outside the class.

fun String.isPalindrome() = this == reversed()

println("level".isPalindrome())

Interfaces and delegation

Implement an interface by delegating to another object with by.

interface Logger { fun log(message: String) }
class ConsoleLogger : Logger { override fun log(message: String) = println(message) }
class Service(logger: Logger) : Logger by logger

Compute a value on first use

lazy runs its block once, the first time the property is read.

val config by lazy { loadConfig() }